Provided is a condensate water trap for a gas furnace that collects and discharges condensate water produced in a heat exchanger and an exhaust pipe. The condensate water trap includes: a first inlet through which the condensate water produced in the heat exchanger is introduced; a second inlet through which the condensate water produced in the exhaust pipe is introduced; a first flow path through which the condensate water coming from the first inlet passes; a second flow path through which the condensate water coming from the second inlet passes; an outlet through which the condensate water introduced through the first and second inlets is discharged; a third flow path into which the residual condensate water passed through at least one of the first and second flow paths but not discharged through the outlet is introduced; and a sensing mechanism that senses if the amount of residual condensate water introduced into the third flow path is greater than or equal to a given amount.